Meristematic Tissues

A meristem is an embryonic tissue primarily responsible for producing more cells that support the growth of the organism. Dicots and gymnosperms have primary and secondary plant bodies that are formed from different meristematic tissues. The primary plant body refers to the plant prior to the commencement of woody (or secondary) growth. Apical meristems occur at the terminal ends of roots and shoots and build the primary plant body. The outermost cells of the root are constantly being worn away and new cells at the apical end of the meristem move

Table 8.4 Primary Types of Vascular Bundles

Bundle Type

Characteristic

Collateral bundles

Phloem is external to xylem. In the case of leaves, the xylem is toward the upper epidermis (adaxial surface) and phloem is toward the lower epidermis (abaxial surface). Collateral bundles are the most frequently encountered bundle type in medicinal plant species (Figure 8.8a and b).

Bicollateral bundles

Phloem is both external and internal to xylem (Figure 8.8c).

Amphivasal bundles (leptocentric bundles)

Xylem completely surrounds the phloem (e.g., Rheum spp., many monocots) (Figure 8.8d).

Amphiphloic bundles (amphicribral bundles)

The phloem completely surrounds the xylem; this type of bundle is rarely seen in medicinal plants (e.g., Rhodiola spp. and many ferns) (Figure 8.8e and f).

FIGURE 8.4 Transverse section of root/rhizome epidermis and hypodermis. (a) Valeriana officinalis rhizome epidermis and hypodermis containing oil droplets, with cortical parenchyma; (b) Valeriana officinalis rhizome epidermis and hypodermis containing oil droplets, with cortical parenchyma. (Images courtesy of Prof. Dr. Reinhard Länger, AGES PharmMed, Vienna, Austria.)

FIGURE 8.4 Transverse section of root/rhizome epidermis and hypodermis. (a) Valeriana officinalis rhizome epidermis and hypodermis containing oil droplets, with cortical parenchyma; (b) Valeriana officinalis rhizome epidermis and hypodermis containing oil droplets, with cortical parenchyma. (Images courtesy of Prof. Dr. Reinhard Länger, AGES PharmMed, Vienna, Austria.)

FIGURE 8.5 Structure of vascular bundles of Tussilago farfara leaf showing fibers, sclerenchyma, phloem, xylem, and line of cambium. (Images courtesy of Prof. Dr. Reinhard Länger, AGES PharmMed, Vienna, Austria.)

forward to replace them. As the old cells disintegrate, a strong protective root cap is formed. Some of the new cells become part of the root cap, but most become part of that portion of the root associated with its growth cell division and elongation (elongation region or meristem).

The secondary plant body refers to tissues that arise when wood and bark are formed from lateral meristems called the vascular cambium and cork cambium, respectively. Secondary growth does not occur in monocots. When viewed in transverse section, cambial cells of meristems appear rectangular, tangentially elongated, thin walled, and arranged in distinct radial and tangential rows. Due to the delicate cell wall, cambial cells may not be discernible in dry medicinal plant parts. Neither apical nor lateral meristems provide diagnostic characters for the microscopist. Details of the development of the vascular and cork cambia in stems and roots and the associated changes in tissue organization are described in the following chapter.

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